Re: [Xen-devel] DomU application crashes while mmap'ing device memory on x86_64

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 30/11/16 19:00, Oleksandr Andrushchenko wrote:
> I traced the problem down to vma->vm_page_prot which
>
> in my case is set as:
>
> vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
>
> This sets additional flags _PAGE_BIT_PSE/_PAGE_BIT_PAT +_PAGE_BIT_PCD
>
> so after that remap_pfn_range makes Xen complain.
>
> (pgprot_noncached(vma->vm_page_prot) == 0x80000000000000b7)
>
> If I change prot to
>
> vma->vm_page_prot = PAGE_SHARED;
>
> (PAGE_SHARED == 0x8000000000000027) then I am able to mmap.
>
> Can anyone please help me understand if this is a valid use-case for DomU
> (pgprot_noncached) and if so why Xen cannot make it?
>
> Thank you,
> Oleksandr

Superpages are not supported in a PV guest.

You can enable the use of 2mb superpages for PV guests by booting Xen
with the "allowsuperpage" command line option, but quite a few features
are broken in combination with PV superpages, and this area has been a
ripe source of security bugs.

Unprivileged guests (i.e. ones without hardware) are not permitted to
make mappings with anything other than a writeback memory type, because
all kinds of chaos can ensue if the guest constructs aliasing mappings
with different cacheabilities, and it is prohibitively expensive for Xen
to track for auditing purposes.

~Andrew

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